Isoprene has a slower polymerization rate than styrene in hompolymerizations which are initiated utilizing organolithium compounds. However, isoprene exhibits a faster rate of polymerization than styrene in copolymerizations which are carried out in nonpolar solvents. As a consequence of this difference in polymerization rate, in batch copolymerizations of styrene and isoprene, the initial portion of the copolymer chain is mainly comprised of repeat units which are derived from isoprene. This is followed by a block of repeat units which are derived primarily from styrene due to exhaustion of the isoprene monomer. The styrene-isoprene rubber which is produced from such polymerizations is, of course, very blocky and also exhibits a very low vinyl content.
Polar modifiers are commonly used in the preparation of synthetic polydiene rubbers which are prepared utilizing lithium catalyst systems in order to increase their vinyl content. Ethers and tertiary amines which act as Lewis bases are commonly used as modifiers. For instance, U.S. Pat. No. 4,022,959 indicates that diethyl ether, di-n-propyl ether, diisopropyl ether, di-n-butyl ether, tetrahydrofuran, dioxane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, trimethylamine, triethylamine, N,N,N',N'-tetramethylethylenediamine, N-methyl morpholine, N-ethyl morpholine, and N-phenyl morpholine can be used as modifiers. The vinyl group content of polydienes prepared utilizing such Lewis bases as modifiers depends upon the type and amount of Lewis base employed as well as the polymerization temperature utilized. For example, if a higher polymerization temperature is employed, a polymer with a lower vinyl group content is obtained (see A. W. Langer: A. Chem. Soc. Div. Polymer Chem. Reprints: Vol. 7 (1), 132 [1966]).
The utilization of such polar modifiers causes styrene to polymerize much faster than isoprene. Hence, the initial portion of styrene-isoprene copolymers synthesized in the presence of such polar modifiers is mainly comprised of repeat units which are derived from styrene followed by a block of repeat units which are derived primarily from isoprene. Additionally, such copolymers have a low 1,4-polyisoprene microstructure content which results in the polymer providing poor tread wear characteristics when utilized in tire tread rubber compounds.
It has not been possible to prepare truly random copolymers of styrene and isoprene in batch polymerizations. Polar modifiers can be utilized to shift the relative rates of styrene and isoprene polymerization. However, standard polar modifiers do not cause styrene and isoprene to polymerize at the same rate. Such a modifier is required to prepare truly random styrene-isoprene copolymers by batch polymerization.